The present invention relates to a liquid flow meter, more particularly a sonic liquid flow meter. Liquid flow meters of the foregoing type measure the rate of flow of a liquid within a conduit (e.g., a pipe) by transmitting a plurality of sonic pulses through the fluid in both an upstream and downstream direction and by computing the rate of flow as a function of the difference in time it takes for the upstream beam to travel through the fluid with respect to the time it takes for the downstream beam to flow through the fluid.
Typical of such flow meters are U.S. Pat. Nos. 3,869,915 (hereinafter the '915 patent) and 3,987,674 (hereinafter the '674 patent) both issued to Joseph Baumoel. In accordance with the teachings of these patents, a sonic beam is transmitted between first and second transducers located adjacent the outer walls of the conduit. The transducers are spaced in such a manner that one transducer is located upstream from the other transducer. In each of these patents, a plurality of sonic beams are transmitted from the upstream to the downstream transmitter and then from the downstream to the upstream transmitter. By measuring the difference in travel time of the upstream and downstream pulses, an indication of flow rate of the fluid in the conduit is provided.
The primary advantage of sonic flow meters of the foregoing type is that they do not require an invasion of the conduit walls. As such, installation of the sonic flow system does not require a shutdown of the system being monitored nor does it require the cutting into the conduits. Additionally, since the sonic flow meter does not require physical contact between the measuring apparatus and the liquid whose rate is being measured, there is no possibility that the sonic apparatus will hinder flow or will be adversely affected by the chemical nature of the fluid being monitored.
While sonic flow meters such as those described in the foregoing patents overcome the drawbacks of standard mechanical flow meters, their accuracy depends on their ability to precisely detect the time interval between the instant at which the sonic beam is transmitted from one transducer (e.g., the downstream transducer) until it is received by the second transducer (e.g., the upstream transducer). In practice, at a flow rate of 30 feet per second the difference between the upstream and downstream travel times for a sonic beam is only about 0.2 percent of the time taken to travel either upstream or downstream. Since the travel times are extremely small (approximately 20 microseconds per inch in water) the difference in the upstream and downstream travel time is only a small percentage of the period of the sonic beam being transmitted (which beam is normally a sinusoidal pulse with an exponential envelope in form). In order to obtain the desired accuracy, it is imperative that receipt of the transmitted sonic beam is detected with reference to the same point of the pulse during each upstream-downstream pair of transmissions. In the foregoing patents, this result was obtained by detecting a specific zero crossing point of each of the sinusoidal sonic pulses transmitted.